1. Field of the Invention
This invention relates to a glass-ceramic having hexacelsian as a primary crystal phase and a magnetic disk substrate using the same.
2. Description of Related Art
Recently, a chemically strengthened glass and a glass-ceramic containing lithium disilicate as a primary crystal phase are used as a magnetic disk substrate. In the case of lithium disilicate glass-ceramics, lithium may diffuse at an end portion of a thin substrate from the substrate through an under-coat, a magnetic film and a protection film and react with a lubrication film.
And also, an alkali-free glass-ceramic is proposed for improving a weather resistance of a glass substrate. In the alkali-free glass-ceramic, however, the polishing with cerium oxide is difficult because the polishing rate is extremely low.
For example, U.S. Pat. Nos. 5,786,286 and 5,910,459 disclose that a glass-ceramic having hexacelsian as a primary crystal phase is used as a magnetic disk substrate. Since hexacelsian is large in the thermal expansion and alkali-free, it is effective to realize the thermal expansion required as the magnetic disk substrate by using such a glass-ceramic. However, such a substrate is not put into practical use due to abnormal expansion accompanied with phase transformation up to now.
In the above two patents, there is disclosed that the abnormal expansion accompanied with the phase transformation of hexacelsian is removed by specifying the glass composition. However, the glass-ceramic is subjected to polishing finish with abrasive grains of aluminum oxide for preparing the magnetic disk substrate. Because, the glass-ceramic is hard and is too low in the polishing rate for abrasive grains of cerium oxide widely used in the polishing industry for the glass substrates (that is, it is impossible to polish with the cerium oxide grains). As described, for example, on column 11, lines 54 to 55 of the above U.S. Pat. No. 5,910,459, the glass-ceramic is subjected to a finishing treatment of lapping with 7 xcexcm alumina slurry and then polishing with 1.5 xcexcm alumina polishing slurry because the hardness of the alumina grain is considerably higher than that of cerium oxide grain.
It is, therefore, an object of the invention to provide a glass-ceramic having a high weather-resistance hexacelsian as a primary crystal phase and further having a high polishing rate capable of polishing at a sufficiently practical rate with abrasive grains of types usually used in industry such as cerium oxide and the like.
It is another object of the invention to provide a magnetic disk substrate made of the above glass-ceramic and having a face of small center-line average surface roughness Ra and excellent characteristics from a viewpoint of thermal expansion coefficient.
According to a first aspect of the invention, there is the provision of a glass-ceramic having a hexacelsian as a primary crystal phase, characterized by satisfying a relation of 0xe2x89xa6I(101)/I(110)xe2x89xa61.5 when a diffraction intensity of (101) face of hexacelsian in an X-ray diffraction is I(101) and a diffraction intensity of (110) face thereof is I(110).
According to a second aspect of the invention, there is the provision of a glass-ceramic characterized by ceramming a precursor glass comprising 43-57 mol % of SiO2, 7-15 mol % of Al2O3, 13-21 mol % of MgO, 4-12 mol % of BaO, 2-6 mol % of K2O and 5-9 mol % of TiO2 through a heat treatment.
According to a third aspect of the invention, there is the provision of a magnetic disk substrate made of the above glass-ceramic.